In electrophotographic image forming apparatuses, such as copiers, printers, facsimile machines, multifunctional machines formed by combining these devices, etc., there is generally provided a fixing device to fuse and fix toner (i.e., developer) onto a recording medium sheet. In such a fixing device, a prescribed temperature often referred to as a fixing temperature is needed for fixing an unfixed toner image, and accordingly is set in advance.
However, the fixing temperature varies depending on the type of toner image and/or recording medium used. Typically, the conditions that cause the fixing temperature to vary include density of toner attracted onto a sheet or the number of isolated toner dots attracted thereto and the like. When relatively large numbers of isolated toner dots are attracted onto a sheet and accordingly a printing rate is relatively high, the fixing temperature generally needs to be higher than when relatively small numbers of isolated toner dots are attracted thereonto, and accordingly the printing rate is relatively low. For this reason, the fixing temperature in conventional image forming apparatuses is set using the most difficult conditions for fixing the toner onto the recording medium.
However, when a fixing operation is executed under such a worst-case scenario even in a situation in which fixing can be carried out easily, the fixing temperature is maintained at an unnecessarily high level, thereby wasting power in a heating system and going against recent energy saving trend.
Further, such a problem relates not only to the fixing temperature of an actual fixing operation but also to that of a start-up temperature for preheating the fixing device executed before starting the fixing operation. Specifically, if the start-up temperature is decreased, energy can be saved by reducing the power to be supplied to the fixing device before start of the fixing operation.
Further, a sleep mode is increasingly employed in an image forming apparatus these days, to be entered immediately after completion of printing as a start-up time period needed before starting up the fixing device decreases. Accordingly, the start-up time period increasingly impacts energy saving.
Japanese Patent No. 3,295,273 (JP-3295273-B) describes a technology capable of changing a start-up time period for starting up a fixing device in accordance with a difference in image resolution between a character mode and a photographic mode. With such a technology, the start-up time can be shortened in the character mode due to its low resolution (i.e., favorableness to fixed condition), thereby further minimizing energy consumption in comparison with a situation in which the start-up time is invariably long.
However, various factors impact the fixed conditions of a toner image beside the resolution thereof. For example, since a dot shape differs depending on the type of gradation sequence processing used, fixing performance is significantly impacted by the type thereof. Accordingly, if the start-up temperature is controlled only based on the resolution of the image as described in JP-3295273-B, no large energy savings can be expected.